“If the parents have a deal — `You can use my car if we put a locator on it, and if you ever get in a bind, I’ll know exactly where you are’ — most teenagers won’t mind that,” said Chris Newton, the chief executive of Securus.

Long contract

Neither Amber Alert nor Securus markets its devices to people who are looking to invade other people’s privacy. What’s more, both firms’ devices require at least a yearlong service contract, which make them somewhat unsuitable for the infidelity market. Why sign up for a year if you only want to track your straying spouse’s whereabouts for the next few weeks?

With that customer in mind, Rocky Mountain Tracking, a GPS firm that mainly sells trackers to companies looking to monitor their fleet vehicles, recently created a monthly rental plan for its personal tracking device, called Ghost Rider. For $99.95 a month, you’ll get the device and a service plan without a contract. You can use the device however you wish as long as you obey the law. (The rules of monitoring vehicles varies by state, but it’s generally considered legal if you’re tracking a car that you own.)

When you’re done with the device, you send it back to Rocky Mountain Tracking.

“We probably rent out 10 to 12 devices a month, and we don’t ask any questions,” said Gary Whitney, Rocky Mountain’s director of sales.

“Occasionally I’ll get a person — usually it’s the wife — who feels guilty about doing it,” he said. “But not so guilty that they don’t go ahead and get the device.”